It is common to support a vehicle with a steel beam or leaf spring which is attached at each end to the vehicle chassis and to an axle near the spring's center. In such an arrangement, when the suspension is loaded, the maximum bending moment in the spring occurs at the point where the axle is attached to the spring and descreases in either direction from that point along the spring. It has long been recognized that to make the most efficient use of the spring material, the spring should be tapered in either direction toward its ends from a point of maximum thickness where the axle is attached. Early examples of such tapered springs are shown in U.S. Pat. No. 129,297. To achieve acceptable deflection characteristics in a tapered spring, it is necessary that it be manufactured to a relatively high degree of accuracy. Unfortunately, the manufacturing process available at the time U.S. Pat. No. 129,297 was issued did not permit economical production of such a spring. Even today, the cost of manufacturing a tapered steel leaf spring is relatively high because it requires the use of special tapered rolling machines. As a result, although tapered springs are generally available today for use with larger vehicles, such as heavy trucks, they are significantly more expensive than conventional flat springs.
Another important factor in the design of leaf spring suspensions is the desired spring rate. "Spring rate," which is defined as the rate of increase of force necessary to deflect the spring with deflection, is a function of the cross-sectional area moment of inertia, the length of the spring, and the elastic modulus of the spring material. In general, the spring must be designed so that it is strong enough to withstand the loads imposed upon it in operation and yet have a spring rate which is low enough to provide acceptable ride qualities. Further, the desired spring rate must be achieved within the particular geometric constraints placed on the suspension, such as the maximum allowable length and deflection of the springs.
To achieve a compromise among these various design and economic factors, designers frequently use "built-up" steel spring assemblies which consist of a number of separate spring leaves diminishing in length from the top of the assembly to the bottom to achieve an overall tapered shape. Normally, the leaves are clamped together at their centers where the axle is mounted but are free to slip longitudinally relative to each other when the spring is deflected. The leaves are usually of constant thickness to reduce manufacturing costs, but use of tapered leaves obtains greater efficiency. As previously mentioned, however, the cost of manufacturing such tapered leaves is relatively high.
One of the principal deficiencies of a built-up steel spring assembly is its weight. Due to the dramatic increase in fuel costs in recent years and the consequent necessity to reduce vehicle weight, designers are examining all major vehicle components, including suspensions, to see if ways can be found to reduce their weight without adversely affecting their cost or performance. In particular, it has been suggested that much lighter and more efficient springs could be made from various state-of-the-art plastic or composite materials rather than steel. Some of these materials are particularly attractive for use in constructing springs with nonuniform cross-sections because of the ease with which they can be molded. For a general discussion of the efforts which have been made to adapt these materials for use in leaf springs, see U.S. Pat. No. 3,586,307, to Brownyer.
In spite of these efforts, plastic and composite springs have not been used commercially for a variety of reasons. It is generally accepted that springs made entirely of plastic would be impractical because of excessive bulk and insufficient resistance to wear and impact. One known plastic spring, sold under the trademark GRAFTEK by a division of Exxon Corp., is made from graphite skins with a glass fiber-reinforced epoxy core. This spring has proven impractical due to high cost and its extreme unidirectional stress-carrying capability (anisotropy). That is, the spring is strong enough in the vertical direction but too weak in the transverse or torsional direction to be usable in common suspensions. Since vehicle springs must absorb cornering loads and high impact loads from rocks and other debris, graphite is unsatisfactory as a component of a viable leaf spring. Graphite, for example, has a tensile strength of only around 800 inch-lbs/in.sup.3. Composites of metal and plastic have been suggested to alleviate some of these graphite problems. The cost of manufacturing these composite springs has thus far been too great, however, to justify substituting them for all steel spring assemblies.